In recent years, there have been proposed and developed various electronically-controlled power steering systems each employing a reversible pump and a hydraulic power cylinder to provide steering assistance. One such hydraulic power steering system has been disclosed in Japanese Patent Provisional Publication No. 2002-145087 (hereinafter is referred to as “JP2002-145087”), assigned to the assignee of the present invention. In the hydraulic power steering system disclosed in JP2002-145087, downstream ends of a pair of hydraulic-pressure lines are respectively connected to left and right pressure chambers defined on both sides of a piston slidably accommodated in a hydraulic power cylinder. On the other hand, the upstream ends of the two hydraulic-pressure lines are connected to respective discharge ports of a reversible pump. The resulting pressure differential applied to the two sides of the piston, produces a steering assistance force. The magnitude and sense of the steering assistance force are determined based on the magnitude and sense of the steering torque applied to a steering wheel by the driver. Also provided is a communication passage or a bypass passage through which the two hydraulic-pressure lines, that is, the left and right pressure chambers, are intercommunicated with each other, when at least one of the reversible pump and the motor both incorporated in the power steering system is failed. A directional control valve (a shut-off valve), such as a two-position, spring-offset, two-way spool valve, is disposed in the communication passage, to block fluid communication between the two hydraulic-pressure lines via the communication passage during normal steering operation. On the contrary, in presence of a power steering system failure, such as a reversible pump failure or a motor failure, the directional control valve is shifted to its open position for fail-safe purposes, so as to permit full fluid-communication between the two hydraulic-pressure lines via the communication passage so that the power cylinder is held in the free state and does not produce an assisting force, and that a manual steering mode is ensured.
However, the power steering system disclosed in JP2002-145087 uses a normally-open, solenoid-actuated two-port two-position spring-offset spool valve as a directional control valve. Thus, in the system of JP2002-145087, an exciting current must be continuously applied to the solenoid of the directional control valve during a normal power steering mode (or a normal power-assist control mode) where the power steering system is normally operating with no power steering system failure. Continuously energizing the solenoid of the directional control valve during the normal power-assist control mode results in increased electric power consumption and increased calorific value of heat generated. From the viewpoint of improved fuel economy of a power-steering-system equipped vehicle, during the normal power-assist control mode the more reduced electric power consumption (more reduced calorific value of heat generated from the solenoid) would be desirable.